1. Field of the Invention
The present invention relates to excimer and molecular fluorine lasers, and particularly to a laser gas mixture and output beam parameter stabilization technique.
2. Discussion of the Related Art
Excimer lasers are used in industrial applications such as optical microlithography, TFT annealing, and micromachining. Such lasers generally include a discharge chamber containing two or more gases such as a halogen and one or two rare gases. KrF (248 nm), ArF (193 nm), XeF (350 nm), KrCl (222 nm), XeCl (308 nm), and F.sub.2 (157 nm) lasers are examples. As industrial processes are driven to the leading edge of technology, the stability of excimer laser output beam parameters should be kept as constant as possible over the lifetimes of the gas mixture and the laser itself. It is also important to maintain high discharge efficiencies for operating the laser at high repetition rates.
The efficiencies of excitation of the gas mixtures and various parameters of the output beams of these lasers vary sensitively with the compositions of their gas mixtures. An optimal gas mixture composition for a KrF laser has preferred gas mixture component ratios around 0.1% F.sub.2 /1% Kr/98.9% Ne (see U.S. Pat. No. 4,393,505, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application). A F.sub.2 laser may have a gas component ratio around 0.1% F.sub.2 /99.9% Ne (see U.S. patent application No. 09/317,526, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application). Small amounts of Xe may be added to rare gas halide gas mixtures, as well (see R. S. Taylor and K. E. Leopold, Transmission Properties of Spark Preionization Radiation in Rare-Gas Halide Laser Gas Mixes, IEEE Journal of Quantum Electronics, pp. 2195-2207, vol. 31, no. 12 (Dec. 1995; see also U.S. patent application No. 60/160,126, assigned to the same assignee as the present application and hereby incorporated by reference).
Any deviation from the optimum gas compositions of these or other excimer or molecular lasers would typically result in instabilities or reductions from optimal of one or more output beam parameters such as beam energy, energy and energy dose stability, temporal pulse width and shape, temporal coherence, spatial coherence, discharge width, bandwidth, and long and short axial beam profiles and divergences. Especially important in this regard is the concentration (or partial pressure) of the halogen-containing species, e.g., F.sub.2 or HCl, in the gas mixture. The depletion of the rare gases, e.g., Kr and Ne for a KrF laser or Xe and a buffer gas for a XeCl laser, is low in comparison to that for the F.sub.2, although these rare gases are also replenished at longer intervals.
It is not easy, however, to directly measure the halogen concentration within the laser tube for making rapid online adjustments (see U.S. Pat. No. 5,149,659 (disclosing monitoring chemical reactions in the gas mixture) and Japanese Patent No. JP 10341050 (disclosing a method wherein optical detection of a halogen specific emission is performed)). Therefore, advantageous methods applicable to industrial laser systems include using a known relationship between F.sub.2 or HCl concentration and a laser parameter. In such a method, precise values of the parameter would be directly measured, and the F.sub.2 or HCl concentration would be calculated from those values. In this way, the F.sub.2 concentration may be indirectly monitored, and optimized.
Previously described methods for laser gas characterization include measuring the spectral width of the laser emission (see U.S. Pat. No. 5,450,436 to Mizoguchi et al.), measuring the spatial beam profile of the laser emission (see U.S. Pat. No. 5,642,374 to Wakabayashi et al.) and measuring other characteristics of the output beam such as bandwidth, coherence, driving voltage, amplified spontaneous emission or energy stability wherein a rough estimation of the status of the gas mixture may be made (see U.S. Pat. No. 5,440,578 to Sandstrom, U.S. Pat. No. 5,887,014 to Das, and U.S. patent applications No. 09/418,052, 09/167,653, 60/124,785, each application of which is assigned to the same assignee and is hereby incorporated by reference).
In the '653 application, a data set of an output parameter such as pulse energy and input parameter such as driving voltage is measured and compared to a stored "master" data set corresponding to an optimal gas composition such as is present in the discharge chamber after a new fill. From a comparison of the data values and/or the slopes of curves generated from the data sets, a present gas mixture status and appropriate gas replenishment procedures, if any, are determined and undertaken to reoptimize the gas mixture.
Another technique uses a mass spectrometer for precision analysis of the gas mixture composition (see U.S. Pat. No. 5,090,020 to Bedwell). However, a mass spec is an undesirably hefty and costly piece of equipment to incorporate into a continuously operating excimer or molecular laser system such as a KrF, ArF or F.sub.2 laser system which are typical light sources used in microlithographic stepper or scanner systems. Yet another technique measures fluorine concentration in a gas mixture via monitoring chemical reactions (see U.S. Pat. No. 5,149,659 to Hakuta et al.), but this method is not suitable for use with a rapid online correction procedure. It is desired to have a precise technique for monitoring gas mixture status that is easily adaptable with current excimer or molecular laser systems and provides rapid online information.
In typical gas discharge lasers such as excimer or molecular fluorine lasers, a constant laser pulse energy is maintained in short-term notwithstanding the degradation of the gas mixture by regulating the driving voltage applied to the discharge. As mentioned above, long term regulation is achieved by gas replenishment actions such as halogen injections (HI), total pressure adjustments and partial gas replacement (PGR). The smoothed long-term stabilization of the gas mixture composition uses a regulation loop where input laser system parameter data are processed by a computer (see the '653 application, mentioned above).
In these typical laser systems, an energy detector is used to monitor the energy of the output laser beam. The computer receives the pulse energy data from the energy detector as well as driving voltage information from the electrical pulse power module. This information is not selective enough since the energy monitor signal is influenced not only by the gas but also by resonator optics degradation or misalignment. The typical operation mode is the so-called energy-constant mode where the pulse energy is kept constant by adjusting the driving high voltage of the electrical pulse power module. In this way one gets constant values from the energy monitor.
A change of the laser status which again can be caused by gas aging as well as by the status of the laser resonator leads to a change of the driving voltage. It is desired to operate the laser at an approximately constant driving voltage level. To achieve this an appropriate smoothed gas regulation procedure is necessary. In the example of an excimer laser, usually the halogen gas component (F.sub.2 in KrF lasers, HCl in XeCl lasers) is depleted whereas the other gases (nobel gases Kr and Ne in KrF lasers, Xe and a buffer gas in XeCl lasers) are usually not depleted. Therefore .mu.HI's or other suitable smoothed gas actions such as low flow rate continues flow replenishment are applied (see the '785 application, mentioned above, and U.S. Pat. No. 5,978,406, hereby incorporated by reference for the gas replenishment techniques provided therein).